Users who open the attack PDFs are infected with a variant of a Windows worm known as “Auraax” or “Emold,” researchers said.

The malicious messages masquerade as mail from company system administrators and come with the subject heading of “setting for your mailbox are changed,” said Mary Grace Gabriel, a research engineer in CA Inc.’s security group. A PDF attachment purportedly contains instructions on how to reset e-mail settings. “SMTP and POP3 servers for … mailbox are changed. Please carefully read the attached instructions before updating settings,” the message states.

In reality, the PDFs contain embedded malware and use the format’s /Launch function to execute that malware on Windows PCs running the newest versions of Adobe Systems Inc.’s Acrobat application or its free Adobe Reader, as well as other PDF viewers, such as Foxit Reader.

The /Launch feature is not a security vulnerability per se, but actually a by-design function of the PDF specification. Earlier this month, Belgium researcher Didier Stevens demonstrated how attack PDFs could use /Launch to run malware tucked into documents.

Two weeks ago, security researchers tracked a new run by the Zeus botnet that used the /Launch flaw to infect PCs.

Adobe has previously declined to answer questions on whether in-the-wild use of /Launch in rigged PDFs would prompt the company to update Reader and Acrobat, although it has said a change to the functionality might “conceivably [be made] available during one of the regularly scheduled quarterly product updates.” Brad Arkin, Adobe’s head of security and privacy, has acknowledged that one possible solution would be to disable the function; currently, it’s turned on by default.

After analyzing the attack PDF, other researchers found that hackers are using Stevens’ tactic of modifying the warning that Reader and Acrobat display. Adobe Reader, for example, displays a message telling users to open only those files they know are safe. In the same Windows dialog box, Reader displays the name of the file about to be launched. According to IBM Internet Security Systems researchers, hackers have modified the warning to simply read, “Click the ‘open’ button to view this document.”

Other security researchers, including those at Paris-based CERT-Lexsi, have also reported on the e-mail bearing rogue PDF attachments. CERT-Lexsi added that the malware’s command-and-control server is located in Korea.

IBM researchers said the malware launched from the rigged PDFs seems to be a version of the Auraax or Emold worms. The worm drops a rootkit onto the compromised PC and tries to copy itself to all removable drives, including flash drives, to spread using the “Autorun” infection tactic made popular by 2008’s Conficker worm.

Staffers at IDG — which is Computerworld’s parent company — have received the malicious messages with attached PDF documents. Those messages can pose as ones from “customersupport@domain name.com,” “support@domain name.com,” and “admin@domain name.com,” where domain name is typically the name of the recipient’s company.

An Adobe spokeswoman today declined to comment on the latest attacks and said the company was still researching the /Launch functionality in Adobe Reader and Acrobat to identify “all possible use scenarios for this particular functionality to ensure we are not breaking any common workflows for our customers.” Adobe’s current advice remains that users configure Reader and Acrobat to stymie such attacks, she added. Adobe’s Web site has instructionsabout how to do that.